<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    div{
        width:200px;
        height:200px;
        margin:10px;
    }

    .a1{
        background:radial-gradient(red,black);
    }

    .a2{
        background:radial-gradient(red,green,black);
    }

    .a3{
        border-radius:50%;
        background:radial-gradient(red,black);
    }
</style>
<body>
    
    <div class="a1"></div>
    <div class="a2"></div>
    <div class="a3"></div>
    
</body>
</html>